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Re: [Phys-L] Sunlight Brighter Than The Sun???



I suspect the fact that the solar radiation is, to any approximation I care about here, isotropic about the sun, but NOT homogeneous, has a huge impact. It means there's a momentum flux from *there* to *here*, and the redirection of that by the lens has to be taken into account when making arguments based on the 2LoT. 

I haven't done the calculation, but I suspect that there's a gaping hole in the spatial power fluxes talked about: are those integrated over all frequencies? Put a lens in front of a monochromatic laser beam, and allow the resulting radiation to thermalize and warm a blackbody object. Can it do it? If the laser had negligible energy near 523 nm, and the object now radiates a little in the green part of the spectrum, is that a violation of 2LoT? 

By the way, I very much appreciate discussions of this sort, and on this topic, as I will be asking (and asked!) just this sort of stuff in a class I'll teach next semester. The original question raised by Timothy Folkerts strikes me as a (1) very good example of questioning a statement, and trying to connect it self-consistently with what we already think we know; and (2) an excellent segue into arguments about (say) Wheeler-Feynman absorber theory and why thermodynamic arguments touch upon *everything*. 


________________________________
From: "Folkerts, Timothy J" <FolkertsT@bartonccc.edu>
To: "'Phys-L@Phys-L.org'" <Phys-L@Phys-L.org>
Sent: Tuesday, August 13, 2013 8:05 AM
Subject: Re: [Phys-L] Sunlight Brighter Than The Sun???


But as you focus the light, the "best" you should be able to do is to effectively have sunlight coming from the angle subtended by the magnifying glass -- looking back you would see the surface of the sun everywhere over the surface of the lens.  No matter what sort of lenses & mirrors you had, you would get light as bright (or dimmer) coming from (less than or equal to) the entire upper hemisphere.  This would produce a net flux less than the flux coming directly from the photosphere. 

Or put another way, if you can get 72 MW/m^2 by focusing sunlight, then you could heat a blackbody placed in that light to ~ 5970 K.  This would be heat flowing from a "cool object" (the sun @ 5780 K) to a warm object (the surface @ 5970 K) -- a clear violation of the 2nd Law of Thermodynamics.


-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of curtis osterhoudt
Sent: Tuesday, August 13, 2013 12:58 AM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Sunlight Brighter Than The Sun???

Take the sunlight which falls on an ordinary magnifying glass at the surface of the earth, and calculate its intensity at the middle of the focused bright spot. Realize that the rest of the area in the glass's geometric shadow is much darker than if the glass weren't there.

I don't understand the problem. 

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